The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein.
The force that drives the substance from one side of the membrane to the other is the force of diffusion.
In order for substances to pass through a cell membrane by simple diffusion it must penetrate the hydrophobic core of the phospholipid bilayer.
The types of molecules that can do this are themselves substantially hydrophobic in nature such as carbon dioxide, oxygen or ethanol.
In the figure below the green triangle indicates a concentration gradient of carbon dioxide. The blue arrow indicates the direction of net flow of carbon dioxide. The carbon dioxide penetrates the phospholipid bilayer without the aid of an intermediary molecule. You should be aware that the relative sizes of the molecules in this figure are not correct. The carbon dioxide molecules are much smaller than the phospholipids.
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